Patterns of binding and neutralizing antibody cross-reactivity to multiple HIV-1 virus strains were compared in seropositive hemophiliacs from Japan and individuals from the U.S. Sera were tested using ELISA assays developed to measure binding antibodies to synthetic peptide analogues representing the variable region 3 principal neutralizing determinant (PND) of the HIV-1 envelope glycoprotein. The peptides correspond to the specific amino acid sequences that have been determined for 4 strains of HIV-1 (MN, SF2, IIIB, and RF). The sera were titered for neutralizing antibodies against the same four strains of virus by measuring the inhibition of viral p24 antigen production in Molt-3 cells. Compared to U.S. sera, the sera from Japan had higher PND binding antibody titers to HIV-1 strains MN, SF2, and RF, and higher neutalizing antibody titers to HIV-1 strains MN and SF2. The higher antibody titers observed in the Japanese sera may be due to different lengths of time from seroconversion or other factors unrelated to country of origin. When PND binding titers were compared to neutralizing titers against the corresponding virus strain, there was a correlation for HIV-1 MN and SF2 but not IIIB or RF. Despite differences in magnitude, the prevalence of high titers of antibodies recognizing HIV-1 MN and SF2 in sera from both countries, as well a the correlation between PND binding and neutralizing titer for HIV- 1 MN and SF2, imply a prevalence of viruses closely related to those strains.